In receivers having antennas within their cases, installing appropriate size antennas for receiving radio waves of target wavelengths becomes more and more difficult because of their limited space due to decreases in their sizes. If the antennas are not proper in size for receiving radio waves of target wavelengths, impedance mismatch, or a reduction in voltage standing wave ratio, occurs at input terminals of receive circuits for receiving signals from antennas. As a result, high signal receive sensitivity is not provided.
Receivers having impedance-matching circuits are proposed to solve such a problem. An example of such receivers is shown in FIG. 6A. An impedance-matching circuit 103 having an inductance L and a capacitor C is connected between an antenna 101 and a receive circuit 105. Another example of such receivers is shown in U.S. Pat. No. 6,219,532.
An impedance of the antenna 101 varies according to a radio wave environment around the antenna 101. Thus, the impedance-matching circuit 103 should be designed using parameters obtained from experiments performed in a radio wave environment in which the receiver is actually used. However, the radio wave environment greatly changes according to slight changes in mounting positions and methods of the receiver. Therefore, precisely setting the radio wave environment for the experiments is difficult. For example, the radio wave environment greatly changes according to variations in the number of electric conductors around the antenna 101 and permittivity. Such variations may occur when a metal bracket is used for mounting the receiver, or resin potting is performed on the receiver for making it waterproof. Furthermore, they may occur when a resin is provided in a space around the antenna 101.
Another problem occurs even when the proper parameters are determined for producing target characteristic, for instance, producing a characteristic curve indicated with a solid line in FIG. 6B, the VSWR of which at a target frequency is small. The problem is that the impedance at the target frequency may not be properly matched due to variations in inductance and capacitance, namely, the characteristic of the impedance-matching circuit 103. To solve this problem, building the impedance-matching circuit 103 with highly accurate components, or expanding a frequency band of the antenna 101 by reducing an antenna gain as shown in FIG. 6C. However, the impedance-matching circuit 103 becomes expensive when the highly accurate components are used, and sensitivity of the receiver is lowered when the antenna gain is reduced.